Block and tackle window balance system is a type of window balance device which has been known and in use for more than 30 years. U.S. Pat. No. 3,358,404 disclosed a typical construction of the block and tackle window balance system. A drawback of the window balance system of U.S. Pat. No. 3,358,404 is that the friction force cannot be adjusted. Ideally, the window balance assembly will be constructed such that the forces exerted on the window sash from the tension device and gravity will balance when the window is partially open. See U.S. Pat. Nos. 3,054,152, 3,055,044, 4,068,406, 4,089,085. As the window is closed from the partially open position, the tension device is caused to stretch, further increasing the amount of force exerted by the tension device. Conversely, as the window is opened further from the partially open position, the tension device moves closer to its un-stretched position thereby decreasing the amount of force exerted by the tension device See U.S. Pat. Nos. 3,054,152, 3,055,044, 4,068,406, 4,089,085.
However an improperly balanced window sash will not stay wherever it is set, i.e., it will “hop” if overbalanced or “drop” if under-balanced. Others have attempted to remedy this problem by designing, manufacturing and stocking numerous models of window sash balance systems to support a widely varying range of window sizes and weights. Frequently, five or more different spring force scale or tension devices are required for a particular window sash balance system to account for windows or window sashes of varying sizes and weights. Making or storing different models of window sash balance system to account for the different weights and sizes of windows or window sashes adds to the overall cost of window balance systems.
Adjusting friction force to control the balance of windows has been used as a method for overcoming the drawbacks found in many non-adjustable block and tackle balance systems. Many have attempted to improve the adjustable friction force block and tackle balance system. Many patents, such as U.S. Pat. Nos. 3,358,403, 3,358,404, 4,517,766, 4,654,928 and 4,697,304, have also attempted to improve on the problems encountered by the non-adjustable block and tackle balance system. However, in U.S. Pat. No. 4,517,766, adjustment of the friction force cannot be easily performed after the block and tackle balance system has been installed in a window because the operating or access space is too narrow to turn the adjust screw. Moreover, in U.S. Pat. No. 4,654,928, it is possible to adjust the block and tackle balance system after the balance system has been installed in a window, but the friction force can only be set to either low or high. Since the friction force is not adjustable over a wide range, there is no distinct or real reduction in the inventory of window balance systems that must be maintained to account for the variety in window sashes and weight. In U.S. Pat. No. 4,697,304, a rivet is used to adjust the friction drag against balance cord, but the rivet cannot be easily inserted into or taken out of the tube after the balance is installed in a window. Therefore, adjusting the friction drag against the cord is not available. None of the prior block and tackle balance systems have overcome the fundamental issues or problems of using preferably one friction-adjustable block and tackle balance system for balancing window sashes and the like of widely varying sizes and weights.